1. Technical Field
The present invention relates to a tunable interference filter, a light sensor and an analytical instrument.
2. Related Art
In related art, a tunable interference filter in which mirrors are oppositely provided on surfaces opposed to each other of a pair of glass substrates is known. In the tunable interference filter, lights are reflected between the pair of mirrors, only the light having a particular wavelength is transmitted, lights having other wavelengths are cancelled out by interferences, and thereby, the light having the particular wavelength is transmitted from incident lights.
Further, the tunable interference filter selects the wavelength of the light having the particular wavelength to be transmitted by adjustment of a gap between the pair of mirrors. For the purpose, of the pair of glass substrates, at least one is processed by etching to form a diaphragm and, for example, driving means such as an electrostatic actuator is provided between the pair of glass substrates. In the configuration, by controlling the driving means, the diaphragm can be displaced relative to the stacking direction of the glass substrates and the light having a desired wavelength can selectively be transmitted.
However, in the case where the diaphragm is formed by processing of the glass substrate by etching as described above, the time taken for etching increases and the manufacturing process becomes complex. Further, in the etching of the glass substrate, etching accuracy is not good, and the film uniformity of the diaphragm may vary and the spectroscopic accuracy may be affected.
On the other hand, a tunable interference filter using a silicon substrate that can reduce the etching time at manufacturing with high etching accuracy in place of the glass substrate is known (for example, see Patent Document 1 (JP-A-2006-23606)).
The tunable interference filter disclosed in Patent Document 1 is a tunable interference filter formed by bonding a fixed substrate and a movable substrate. On the fixed substrate, two cylindrical recess parts are formed on the surface facing the movable substrate, and a fixed reflection film and a conducting layer are formed in these recess parts.
Further, the movable substrate is formed by a conducting silicon substrate, and includes a movable part provided nearly at the center of the movable substrate, a support part that displaceably holds the movable part provided on the outer circumferential part of the movable part, and an energization part that energizes the movable part. Furthermore, the silicon substrate does not have light transmittance, and a light transmission part with a cylindrical inner circumferential surface is formed nearly at the center of the movable part, and glass is inserted and fitted in the light transmission part. In addition, a movable reflection film is formed on the surface of the movable part facing the first recess part and facing the fixed substrate.
In the tunable interference filter as disclosed in Patent Document 1, when the support part bends, the movable part displaces toward the fixed substrate side, and the support part and the movable part bend to be convex toward the fixed substrate side. Accordingly, stresses acting in the surface directions of the substrates are different between the substrate facing surface side facing the fixed substrate of the movable part and the incident surface side into which the light opposite to the substrate facing surface enters.
Specifically, at the incident surface side closer than the center position in the substrate thickness direction of the movable part, a force of contraction in the in-plane direction is applied, and the contraction force increases as the location separates farther from the center position. On the other hand, at the substrate facing surface side closer than the center position, a force pulling in the out-of-plane direction is applied, and the pulling force similarly increases as the location separates farther from the center position.
Accordingly, the glass inserted and fitted in the light transmission part is subjected to large lateral pressure as it separates farther from the center position at the incident surface side. Further, at the substrate facing surface side of the glass, the holding force for holding the glass decreases as it separates farther from the center position. Therefore, the glass is subjected to a force to be pushed down toward the fixed substrate side as a whole, and may project toward the fixed substrate side.
If the glass projects toward the fixed substrate side, the gap distance between the movable reflection film and the fixed reflection film varies. In this case, even when the gap distance between the reflection films is adjusted for extracting the light having the desired wavelength using the tunable interference filter, the wavelengths different from the wavelength as the target are extracted, and a problem that the spectroscopic accuracy of the tunable interference filter becomes lower arises.
Further, when the glass projects, the glass surface facing the fixed substrate tilts, and there is another problem that it may be impossible to maintain the fixed reflection film and the movable reflection film in parallel. In this case, the lights having wavelengths different from the wavelength as the target are extracted, and there are problems that the resolution of the transmitted wavelengths becomes lower and the spectroscopic accuracy of the tunable interference filter becomes lower.
As described above, in the tunable interference filter in related art as disclosed in Patent Document 1, there has been a problem that, when the movable part is displaced, the glass projects and it may be impossible to maintain good spectroscopic accuracy.